The invention relates to a working ship for carrying out offshore operations, in particular for preparing and/or exploiting extracting sites of natural resources, such as oil and gas.
During the offshore extraction of natural resources, there is an increasing need for exploitation of extracting sites that are located relatively far away front the shore and/or at a relatively great depth. According to present insights, it is believed that about 65% of the oil supply to be extracted offshore is located in regions relatively far away from the shore where the sea floor is located at a depth of at least 2000 m.
When preparing and exploiting such extracting sites, preparation and exploitation operations have to be carried out on the extracting site. When preparing and exploiting an oil extracting site, these operations consist in, among other things, the steps of placing of a valve on the sea floor, providing a riser pipe construction between the valve and the sea surface, drilling the oil supply in the soil and making it ready for production, extracting oil from the soil, optionally storing and/or processing oil, and discharging the extracted oil.
Because of the relatively great distance from the shore and/or the relatively great depth of the sea floor, it has been found to be a problem to prepare and exploit such extracting sites economically. When the extracting site is located at a relatively great distance from the shore, for instance stores and personnel can be transported by air only to a very limited extent. Consequently, stores and personnel have to be transported relatively fast over the sea to the working site, and on the working site sufficient storage capacity has to be available. When the working site is located at a relatively great depth, it is not possible to provide on the working site a working platform supported by the sea floor.
It has already been proposed that operations for preparing and exploiting such extracting sites be carried out by using a so-called xe2x80x9csemi-submersiblexe2x80x9d. Such a construction comprises a rectangular deck placed on legs. The legs are connected by using floats located below the water surface. Such a semi-submersible is adjustable by varying the floating power of the floats between a floating position in which the deck is located at a relatively great distance from the water surface and a semi-submerged position in which the deck is located relatively close to the water surface. In the floating position, the semi-submersible is transported to the extracting site to subsequently function, in the semi-submerged position, as working platform. It is a disadvantage of such a semi-submersible that it cannot sail independently to a faraway working site but has to be towed to the extracting site by a tugboat. Furthermore, the sailing speed of such a construction is limited so that transport to such a faraway extracting site takes too long. Moreover, the center of gravity of such a construction is located relatively high so that even if few stores are carried along on the deck there is a risk of capsizing. The risk of capsizing is even increased during the adjustment from the floating position to the semi-submerged position.
U.S. Pat. No. 3,837,309 describes a semi-submersible construction having the features of the preamble of claim 1. The construction is capable of being moved under its own power and has a block-shaped, rectangular hull.
FR 1 366 164 describes a semi-submersible having a block-shaped storage tank that, during transport, is used as a hull.
It has also been proposed that the preparation and exploitation of such extracting sites which are far away and/or located at a great depth be carried out by using a working ship with a conventional hull provided with driving means. Such a ship having an elongate hull is described in GB 2 150 516. It is a disadvantage of such a ship that when lying still on the extracting site it follows wave motions of the water surface too much. Consequently, the deck is often not stable enough to carry out the preparing or exploiting operations. This leads to a greatly decreased productivity.
It is an object of the invention to provide a ship that does not have the above disadvantages. To this end, a working ship according to the invention comprises a hull provided with driving means and a deck, which hull is temporarily submersible, and which deck is adjustably connected at an adjustable intermediate distance to the hull by connecting means so that the working ship is adjustable between a floating position in which the deck is located near the hull and a semi-submerged position in which the hull is located substantially below the water surface and the deck is located above the water surface at a distance from the hull.
Preferably, the hull is of elongate shape, having a length to width ratio of more than 3:1. More preferably the hull has a length to width ratio ranging from approximately 4:1 to 5:1. Such a hull of elongate shape provides for a high sailing speed at relatively low engine power and fuel consumption. Surprisingly it has been found that, both in the floating position and in the working position, the elongate hull provides for ample stability. Due to its elongate shape, the longitudinal hull can be positioned with its longitudinal axis substantially transverse to the waves and/or substantially parallel to the wind. This greatly improves operating conditions in the submerged or floating position compared to a waking vessel with a substantially square hull, having a length to width ratio of approximately 1:1. Also, during transition from the floating position to the submerged position, such positioning can improve stability.
The effect thus achieved is that in the floating position the working ship can sail to the extracting site independently, without a risk of capsizing, fast, and provided with sufficient stores, while in the semi-submerged position it is sufficiently stable on the extracting site. In the semi-submerged position, the surface of the working ship located near the water surface is relatively small, while a great part of the total weight of the working ship is located below the water surface. Consequently, the deck, even when much wind and/or high waves are present, will be sufficiently stable to enable the operations to be carried out. To increase the stability, the hull, in the semi-submerged condition, can comprise more than 50%, preferably more than 60% of the total weight of the working ship. For an optimum stability, the center of gravity of the working ship in the semi-submerged condition is located near or below the water surface.
It is observed that when in this context reference is made to a hull, this is a single hull, that is to say a hull forming one floating body. It is further observed that the working ship can also be used on sites other than extracting sites, for instance to lay pipelines on the sea floor. Consequently, reference will hereinafter be made to the working site of the working ship.
Further elaborations of advantageous embodiments of the working ship are described in the subclaims.